1. Field of the Invention
The present invention relates to a multilayer piezoelectric resonator making use of a higher-order vibration mode which is to be used, for example, in an oscillator, filter, etc.
2. Description of the Related Art
A conventional piezoelectric resonator 40 using a thickness extensional vibration mode is shown in FIG. 1. In this piezoelectric resonator, a vibrator portion 40a is formed by a piezoelectric thin-film layer 21 (having thin-film electrodes e1 and e2 laminated on opposite sides of a piezoelectric thin film 21a) and an insulating thin film layer 22. Another conventional piezoelectric resonator is shown in FIG. 2. In this resonator 50, a vibrator portion 50a is formed by laminating an insulating thin film layer 22, a piezoelectric thin-film layer 21, and an insulating thin film layer 22. As used herein, the term xe2x80x9cthin-filmxe2x80x9d refers to films having a thickness in the range of about 0.01 to 100 xcexcm.
In the conventional piezoelectric resonators 40 and 50, large electro-mechanical coupling coefficients are obtained only in the relatively lower-order vibration modes such as the fundamental mode, second mode (second harmonic mode), etc. of the thickness extensional vibration mode. Therefore, when high resonance frequencies are desired, the thickness of the vibrator portions 40a and 50a must be reduced because the resonance frequencies are inversely proportional to the thickness of the vibrator portions 40a and 50a. For example, in the piezoelectric resonator 40 of FIG. 1, when zinc oxide (ZnO) is used for the piezoelectric thin film 21a, the thickness of the vibrator portion 40a must be made as thin as about 4.7 xcexcm in order to realize a resonance frequency of 650 MHz. As a result, the mechanical strength of the vibrator portion 40a decreases and the vibrator portion 40a becomes easily damaged. Further, in the conventional piezoelectric resonators 40 and 50, because the ratio of the thickness of the thin-film electrodes e2 and e2 to the thickness of the vibrator portions 40a and 50a increases, there is also a problem that the damping is increased and the mechanical Q of the resonance is reduced.
The present invention provides a piezoelectric resonator which can effectively operate in a higher-order vibration mode with a high resonance frequency while maintaining the mechanical strength of the vibrator portion.
According to a first aspect of the invention, a piezoelectric resonator using a thickness extensional vibration mode comprises a vibrator portion made up of n layers of piezoelectric thin-film, n being an integer greater of equal to 2, each of the piezoelectric thin-film layers being separated by a respective insulating thin-film layer, the distance d of the nth piezoelectric thin-film layer from a first side of the vibrator portion, being determined by the formula
d=t(2mxe2x88x921)/2n
wherein t is the thickness of the vibrator portion and m is an integer less than or equal to n.
According to a second aspect of the invention, the piezoelectric resonator is used in conjunction with a voltage source, the combination comprising:
a piezoelectric resonator using a thickness extensional vibration mode, the piezoelectric resonator comprising a vibrator portion made up of n layers of piezoelectric thin-film, n being an integer greater of equal to 2, each of the piezoelectric thin-film layers being separated by a respective insulating thin-film layer; and
a voltage source for applying alternating voltages to the piezoelectric resonator in such a manner that the predominant vibration mode excited in the piezoelectric resonator is a higher-order vibration mode.
According to a third aspect of the invention, the invention comprises a method for exciting a piezoelectric resonator in a thickness extensional vibration mode, the piezoelectric resonator comprising a vibrator portion made up of n layers of piezoelectric thin-film, n being an integer greater of equal to 2, each of the piezoelectric thin-film layers being separated by a respective insulating thin-film layer, the method comprising:
applying alternating voltages to the piezoelectric thin film layers in such a manner that the predominant vibration mode excited in the piezoelectric resonator.
In all three aspects of the invention, the vibrator portion preferably has a thickness t equal to half the first (basic) resonance wavelength of a thickness extensional vibration mode of the resonator. The resonator is excited in a nth (second or higher) higher-order vibration mode by applying appropriate alternating voltages to the piezoelectric thin-film layer. Particularly, the alternating voltages are applied in such a manner that the vibrations induced in adjacent piezoelectric thin-film layers are opposite in phase. This can be done, for example, by applying alternating voltages of opposite phase to the adjacent piezoelectric thin-film layers when the polarization direction of those layers is the same. Alternatively, alternating voltages of the same phase can be applied to adjacent piezoelectric thin film layers when the polarization direction of those layers is opposite to one another.
Each of the piezoelectric thin-film layers is preferably formed at a respective nodal point of the nth higher-order vibration mode. As a result, the first (basic) vibration mode or lower-order vibration modes lower than the nth mode disappear or attenuate by canceling each other due to the interference of opposite phase mechanical vibrations of the piezoelectric thin-film layers, and only the nth higher-order vibration mode is predominantly excited. As a result, the electro-mechanical coupling coefficient of the piezoelectric resonator is large and a sharp resonance curve can be obtained.
Further, because the thickness t of vibrator portion) is equal to one half of the resonance wavelength of the basic vibration mode, high resonance frequencies of short wavelengths can be obtained for a given thickness of the vibrator portion. This makes it possible to produce vibrations having higher frequencies without degrading the strength of the vibrator portion.
The piezoelectric thin-films and insulating thin film layers are preferably composed of materials where the temperature coefficient of the elastic constants is of opposite sign. As a result, the temperature coefficient of the elastic constants of the piezoelectric thin-film layers and insulating thin film layers offset one another, thereby stabilizing the frequency-temperature characteristics of the piezoelectric portion
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.